Your situationj seems to be a result of you doing stuff to "help" that backfire. So stop. Find a way to stop. They are not your kids, they're your grandkids. Make your son stand up and resolve it. You go on your vacation and understand that this year might be the year you go without the grandkids. Find a way to make your peace with it. Your son MUST find a way to make HIS peace with it also, and having you worried about it and wanting things from him can only make HIS blood pressure go up as well.
Your position in the kids' lives is ONLY through your son's rights, and so you must rely upon him to do the right thing by them. If he is not doing that, nagging him will not make it happen. Doing it for him will not make it happen (this is one of those things that you, as a mother, can NOT do for him!)
You need to cut the cord, as hard as it might be, and let him do this one on his own.
I do recall your previous posts, how he went through a very difficult time, was he the one who was in an accident and in a coma for a while? I remember some serious trouble that you were helping him through, and you were proud that he'd improved.
So now he's improved. Time to cut the cord.
TRULY, you will not be able to do this for him... and your presence there (in the middle of the court issues) can only make it worse, make it harder for him to figure out what's the OBJECTIVELY right thing to do for the kid... etc.